An experimental study was conducted to investigate fuel injection characteristics through swirl injectors when the fuel was heated to very high temperature conditions. Three swirl injectors with different orifice diameters and swirler geometries were used to measure the flow coefficient (${alpha}$) for the injection pressure ranges between 3 and 10 bar and the fuel temperature from 50 to $270^{circ}C$. The results showed that the variation characteristics of ${alpha}$ with respect to cavitation number ($K_c$) were highly dependent on both the orifice diameter and the swirler geometry. In addition, the characteristics of ${alpha}$ variation with respect to AR, the area ratio of the flow through the swirler and the orifice, has revealed that the effect of boiling is retarded but the slope of decreasing ${alpha}$ after the boiling effect is present tumbles as AR increases.